PROJECT SUMMARY The proposed study (CAIT: Community-based Adaptive Autism Intervention for Toddlers) aims to determine the most optimal sequence of interventions for improving the social- communicative, language and cognitive outcomes of toddlers with autism spectrum disorder (ASD). The target population consists of 2-3 year -old children with ASD who receive community based, publicly funded early intervention in two authentic educational settings in East Harlem and the Bronx, NY known as New York Center for Infants & Toddlers (NYCIT). An expected 300 toddlers with ASD will participate with their community-recruited paraprofessional teaching assistants (TAs) and group leaders (GLs). The study aims to construct the most effective one-year, two-phase, adaptive intervention, in which intervention is individualized based on a child's initial response to intervention. Phase 1, from program entry to either 6 or 12 weeks (randomized), involves 60 minutes daily of an evidence based social communication intervention, JASPER (Joint Attention, Symbolic Play, Engagement & Regulation) delivered 1-on-1 to the child by the trained TA. At end of Phase 1, response to intervention is rated by the GLs for slow or fast improvements of joint engagement (a core deficit in the early development of children with ASD, and a significant indicator of good progress toward improving social communication and language). In Phase 2 (to week 24), toddlers responding slowly are re-randomized to continue with JASPER for 60 minutes per day or augment treatment with direct, structured teaching of social communication targets for 30 minutes and JASPER for another 30 minutes per day. Toddlers responding quickly are given 30 minutes of JASPER and 30 minutes of jasPEER (JASPER with a peer) to further improve socialization and social communication. This study will employ a Sequential Multiple Assignment Randomized Trial (SMART) design to evaluate how best to improve social communication outcomes in toddlers with ASD over two treatment phases. The primary effectiveness aim is to determine among slower responders, the effect of augmenting JASPER with structured teaching vs. continuing JASPER on primary (social communicative utterances) and secondary outcomes (joint engagement and play). Secondary effectiveness aims focus on (i) determining the best time (week 6 or 12) to identify a child as a slower responder, and (ii) examining whether child un-engagement at baseline moderates the effect of measuring response at week 6 or 12, and whether joint attention skills in Phase 1 moderate the effect of Phase 2 treatment among slow responders (JASPER plus structured teaching vs. continuing JASPER). Finally, an implementation aim will examine barriers and facilitators to intervention adoption by NYCIT (by assessing implementation fidelity, refining our external support to center leadership to reliably assess child response, train new staff, and coach TAs to implement intervention components with fidelity, and to examine the degree to which NYCIT leadership sustains the interventions).